The conventional wisdom that the spiral groove bit is irreplaceable is being overturned by the efficiency of the straight groove bit. In some applications, straight grooves are becoming a better tool choice than twist drills.
The forerunners of modern straight drills are The Die Drills for short size, good rigidity and strong tip, which are suitable for drilling hard steel materials with low spindle speed and small feed rate. Its high strength and high rigidity enable it to process holes with good straightness and roundness. Die drill bits can achieve good machining results because they are designed to be machined with metals that produce short chips. Chip control is still a key problem in the machining and application of straight groove bit. Straight channel drills work best for materials that do not produce long, whiskle-like chips, such as cast iron, powder alloys (P/M), and silicon-aluminum alloys with medium to high silicon content (6% or more).
However, with the progress of machine tool technology, coolant transport technology and tool geometry design, straight groove drilling processing applications are expanding. The conventional way of thinking is that the spiral groove of the drill bit is essential for chip removal, and the straight groove drill can only be used to drill shallow holes with depth of 2D to 3D (D is the aperture) in aluminum and cast iron. However, the development of high-pressure cooling system and higher spindle speed of the machine tool make the chip breaking and chip removal performance of the straight groove drill with internal cooling greatly improved.
Under suitable conditions, straight groove drill bit can show better performance than spiral groove drill bit. Straight groove bits have the advantage of being able to produce holes with higher roundness and straightness, and they can cut faster than standard cobalt or HSS bits. For example, the diameter of 0.394 "(φ10mm) Guhring RT150GG straight groove drill instead of twist drill processing aluminum, twist drill cutting speed of 50sfm (15m/min), feed 0.005ipr (0.127mm/r); The straight groove drill can cut at 800sfm (245m/min) and feed at 0.007 IPR (0.178mm/r). The cutting time of straight groove drill is only 1/10 of twist drill.
Part of the reason for this significant efficiency is based on the basic design of the bit -- the straight groove provides a direct path out of the chip discharge hole. However, due to the lack of a positive rake Angle on the cutting edge of a straight groove bit, its chip coiling and chip breaking capabilities are poor, especially when machining more ductile materials.
Cutting speed, especially high enough cutting speed, is another factor in the successful application of straight channel drills. The faster the cutting speed, the smaller the chip is formed. It is through the faster spindle speed rather than the higher feed rate that the straight groove drill achieves high productivity.
Many high-performance, high-rate twist bits have considerable chamfering on the cutting edge, which requires that the bit feed into work exceed the chamfering size of the cutting edge so that the cutting edge can cut the workpiece material rather than just push it. For some processing applications and materials to be machined, the twist drill's cutting edge chamfering works well, and these twist drills are usually drilled with a higher feed rate and lower cutting speed than straight groove drills.
Take the drilling process at Saturn, a carmaker. Bits used by the company were previously machined at a cutting speed of about 500sfm (150m/min). Then they decided to do a cutting test to determine how high the bits would run. They found that when the rotational speed was increased, the tool life was extended, the machining cycle was shortened, and chip removal efficiency was improved. With uncoated solid carbide bits, cutting speeds of up to 1000sfm (300m/min) are achieved, while tool life and hole quality are significantly improved.
By using the right combination of coolant pressure and cutting speed, straight channel drills can also effectively process workpiece materials that do not necessarily produce short chips. For example, the effect of machining 1144 stress resistant steel with straight groove drill is very good, chip breaking is smooth and small, and no clastic nodules are formed. The drilling bit was H.A.M. Recision 294-0500 uncoated inner cooling straight groove bit (diameter φ5mm), the drilling depth was 48mm, the cutting speed was 185sfm (63m/min), the feed rate was 0.003ipr (0.076mm/r), Coolant pressure is 1500psi. The content of manganese sulfide in 1144 alloy is high, which is beneficial to chip breaking. To process this material successfully, however, a high coolant pressure is required (experience suggests that the coolant pressure should be above 600psi, preferably 1000psi, 1200psi or higher).
In addition to chip control, lumps can be a problem for straight channel drills. This is because the removed workpiece material passes through the cutting edge and is pushed towards the bit tip as it exits the center of the straight slot drill. One way to avoid the formation of nodules is to use a smooth, high finish bit; The use of coated bits can also help prevent clastic formation. For example, MoS2 coated bits, widely used in aluminum processing, have similar properties to Teflon and do not bond easily with other materials, making them excellent for applications where clastic formation prevention is required.
The tip structure of straight groove drill allows it to grind 2 edge belts at the outer edge of each straight groove. Because the drill bit has 4 contact points with the workpiece, the drill bit is more stable in processing, thus improving the machining accuracy.
The large tip Angle used by straight groove bits also helps to improve its machining performance. For example, h.A. M. Recision's standard straight groove bit has a 140° apex Angle, similar to a split crown, which can be drilled into the hole with very little push force. In addition, the high tip Angle ensures that the entire diameter of the bit engages rapidly with the workpiece material, minimizing the chance of bit "running off".
In many processing cases, it's best to use a its apex Angle is greater than the carbide straight flute bit apex Angle of guide bit drilling (center) work out a starting hole (hole), so that can ensure that the formal processing of straight groove bit drilling center (chisel edge) first into the hole, in order to avoid the brittle larger carbide cutting blade first contact with the workpiece caused by the collapse edge. Another strategy for drilling with straight grooves is to simply locate the hole before drilling. Since even the machined workpiece surface may have defects, the operator simply touches the pre-hole with the drill bit to determine its position. As long as the edge of the bit touches the workpiece first, the bit will not "stray", and the positioning hole can act like a drill sleeve.
The results show that the cutting speed of HSS spiral groove bit is 98sfm (30m/min), the feed rate is 7.66IPm (195mm/min), and the machining quality of hole reaches IT12 grade by using external cooling method. The solid carbide spiral groove bit was designed with a cutting speed of 295sfm (90m/min), feed rate of 34.5ipm (876mm/min), internal cooling of 440psi/2.4-gpm, and hole machining quality of IT9. The straight groove bit used a cutting speed of 425sfm (130m/min), feed rate of 33.3ipm (845mm/min), and internal cooling of 735psi/3.2-gpm to achieve IT8 hole quality.
Many people refer to all straight groove drills as "G drills", but this is not exactly true, in fact the "G drills" are just Accuromm USA Int. A trademark for a type of straight groove drill with a tail edge that squeezes the hole wall.
Comparing the hole wall finish achieved by various hole machining tools, the typical HSS bit is about 125-rms, cemented carbide spiral groove high efficiency bit is about 63-rms, three-groove reamer is about 32-rms, and G-drill can achieve 16-rms or less, which is the finish finish level.
Accuromm designs a variety of drill bits to process holes at different quality levels. The COMPANY's G-drill typically meets hole machining requirements for H9 tolerances, while Accuromm's G-7 bit incorporates both drilling and reaming functions to meet H7 tolerances. For example, Accuromm provides an example of drilling a hole in aluminum using a straight groove G drill instead of a twist drill. The G drill has a diameter of 0.394 "(φ10mm), a cutting speed of 330ft./min (100m/min), and a feed rate of 0.008ipr (0.20mm/r). The machining accuracy of hole is improved significantly, and the aperture error is reduced from ±0.0032 "(0.08mm) to ±0.0012" (0.03mm). At the same cutting parameters, the aperture error can be reduced to ±0.0008 "(0.02mm) if machined with a G-7 bit.
Because of the higher quality of the straight groove drilling, it is also of great benefit to other hole processing processes. According to Accuromm, when tapping holes machined by a straight groove drill, its superior dimensional accuracy can increase tap life by approximately 50%.
The improvement of cutting tool performance almost always requires a narrower range of machining objects. Although the straight-channel drill is far from being a universal tool, its use is growing. The popularization and application of straight groove drill can improve the processing efficiency and improve the quality of workpiece.
An integral carbide straight slot step Drill "MuilT-Drill" manufactured by H.A.M. Recision uses TiAlN coating and internal cooling channel design for drilling, step drilling and chamfering in short chip aluminum alloy materials.
Competitive Carbide Int. The company produces the "Insatiable G - Spot" special composite cutting tools will be the company's G - Spot straight flute drill with the inner cavity milling cutter is an organic whole, with a cutting tool geometry of the workpiece can be completed through a variety of processing. The tool can complete 9 machining operations in total, including drilling, OD chamfering and countersink, ID chamfering and countersink, OD turning (pre-machining of threaded workpiece), etc. Due to the large contact area between the tool and the workpiece, and the 4-point contact between the bit outer diameter and the workpiece, the two-piece tool helps maintain the workpiece position and provides additional clamping effect on the workpiece. Since the straight groove drill has two edge belts, the cutter can not only drill holes, but also squeeze the holes. This means that the hole is smoother and more rectilinear than a conventional bit with a single cutting edge, allowing it to better meet specific hole size requirements. The Insatiable G-Spot composite tool can replace at least 3 to 4 tools, which requires 3 tool changes to complete 4 machining operations if the original tool is used. Therefore, the use of multifunctional composite cutting tool, to improve the processing efficiency, shorten the processing time is of great benefit.